Filament Density: Convert Spool Weight to Length

Filament is sold by weight but consumed by length. Every practical question about a spool, how much of a model it will produce, whether a print will finish, what a part actually costs, comes back to converting between those two units. The conversion depends on one property that varies by material: density.

Density is the bridge between weight and length

Density is mass per unit volume, normally given in grams per cubic centimetre on a material data sheet. Filament is a cylinder, so its volume is the cross sectional area multiplied by its length. The area comes from the nominal diameter, which is almost always either 1.75 mm or 2.85 mm depending on the extruder the spool was made for.

To get length from mass, divide the mass by the density to obtain volume, then divide that volume by the cross sectional area. Working the other direction, a slicer reports the length or volume a model needs, and multiplying by density gives the grams the job will consume. Do not carry a value learned for one material over to another. A denser material yields fewer metres per kilogram, so the same spool weight produces noticeably less printed length.

Do not guess densities, and do not assume a material has one. Two mainstream vendors publish different figures for plain unfilled PLA: 1.24 g/cm³ for Prusament PLA against 1.17 g/cm³ for PolyLite PLA. On a 1 kg spool of 1.75 mm filament that is the difference between 335 m and 355 m, before any filler enters the picture.

Filled and composite grades widen the gap much further, because filler loading is not standardised. Carbon fibre and glass fibre reinforced grades do not share the density of the resin they are based on, and Prusa’s documentation on composite filaments makes the same point about their other properties: a filled grade is its own material, not a tinted version of the base polymer, with its own dimensional stability, impact resistance and layer adhesion. Read the figure from the data sheet for the exact spool.

To run the arithmetic without doing it by hand, the site’s filament weight and length calculator takes gross spool weight, tare weight, material density and nominal diameter and returns net mass, remaining metres and printable volume. For the density figures themselves and how they differ between the common materials, see PLA, PETG and ABS compared side by side.

Net weight is not spool weight

Spec sheets quote net filament weight, but the object on the shelf includes the spool itself, and the spool is a substantial fraction of what the scale reads. Prusa publishes the breakdown for its own: two injection-moulded flanges at 81.5 g each and a 39.5 g cardboard centre tube, about 202 g in total. Skip that subtraction on a 1 kg PLA spool and the length estimate comes out roughly 68 m high, which is enough to start a print that cannot finish.

Empty spool weight varies widely between vendors and between cardboard and plastic construction, so one brand’s tare is not another’s. Some manufacturers print the figure on the spool. If yours does not, weigh one spool after it runs out and keep the number for that brand.

Moisture changes how a material prints

Several common polymers are hygroscopic, meaning they absorb water from the air. Nylon and TPU are the usual problem cases, PETG is affected, and even PLA degrades in humid storage. Water absorbed into the filament flashes to steam at the nozzle. The visible symptoms are popping and hissing during extrusion, a rough or foamed surface, stringing that no retraction setting fixes, and weaker layer bonding.

Drying reverses this, but only within limits. Each material has a recommended drying temperature from its manufacturer, and that temperature must stay safely below the material’s glass transition point or the spool will soften and fuse to itself. Look the temperature up rather than reusing one that worked for a different polymer: in a forced-air oven Bambu Lab’s published figures run from 50 °C for basic PLA to 75–85 °C for the PA6-CF and PAHT-CF nylon grades, and drying temperatures and times by filament type sets them out material by material alongside the methods that hold a temperature accurately enough to use them.

Storage is the cheaper fix

Drying is a repair. Storage is prevention. Sealed containers with fresh desiccant keep opened spools usable for far longer than open shelving in a garage or basement. Desiccant is not permanent, so regenerate or replace it on a schedule. If you print continuously with a sensitive material, feeding directly from a sealed dry box is more reliable than drying a spool that has already picked up water.

Bambu Lab’s storage guidance gives the scale of the problem: typical indoor air sits at 45% to 65% relative humidity, most filaments only stay dry below 20% RH, and freshly dried filament left out at around 55% RH picks up enough moisture to affect print quality within two to twelve hours depending on the material. That is a shorter window than most people assume, and it is why an opened spool on an open shelf is exposed rather than stored. Anyone setting up storage for the first time will find the practical starting point in what to buy for a first filament purchase.

Common mistakes

Assuming all filament of a given diameter yields the same length per kilogram. Estimating remaining material by eye rather than by weight. Drying at a temperature copied from an unrelated material. Treating a resealed bag with exhausted desiccant as dry storage. Blaming a printer for extrusion defects that are actually a wet spool.